The present invention relates to a photochromic compound, and more particularly relates to a fulgide type photochromic compound advantageously used for optical recording and copying media.
Fulgide compounds are widely known as one of typical photochromic compounds as disclosed, for example, in Japanese patent publication Sho. 60-52150 and U.S. Pat. No. 4,220,708. In chemical structure of a fulgide compound, two methylenecarbons of succinic anhydride or its derivatives are each accompanied with an alkylidene group. When irradiated with a light of a constant wave length, a fulgide compound exhibits change in chemical structure and, on irradiation with a light of a particular wave length, resumes its initial chemical structure.
Utilizing such a photochemical property, fulgide compounds are generally used for optical recording and copying media. When used for such applications, a photochromic compound is required to have the following functional properties.
(i) High durability against repeated use, i.e. possibility in repeated recording and erasing of information.
(ii) Good thermal stability in darkness, i.e. reliable maintenance of colored and uncolored form.
(iii) High long wave length sensitivity, i.e. fitness to semiconductor lasers.
(iv) Non-destructive reading.
It is reported by H. G. Heller that a photochromic compound having high long wave length sensitivity can be obtained by changing the heteroaromatic structure of a fulgide compound from furan via thiophen to pyrrole as follows; ##STR2##
This change in heteroaromatic structure makes the maximum absorption wave length rise up to 600 nm. Despite this merit, high aromatic character of the pyrrole tends to degrade thermal stability of the colored form of the compound [see A. Kaneko et al, Bull, Chem, Soc, Jpn., 3569 (1988)].
For effective use of a photochromic compound, the compound is required to suffice the above-described three functions (i) to (iii) together. Change in heteroaromatic structure proposed by Heller cannot assure provision of such an excellent photochromic compound.